Thanks to you and
@KAZSoliloquy giving insight (I really enjoyed Joker unlike KAZ but respect his opinion) because that definitely falls in line with what I was thinking. As I was watching Taxi Driver, one thing that really stood out to me was that Bickle seemed to have, in the early going, way better coping mechanisms than Fleck had. We see the similar isolation and disaffected mentality but Fleck appears sadly ill-equipped to function in the environment he is in, while Bickle actually seems to do things to actively make his life more palatable.
To give the example, Fleck is on a lot of meds in order to try to make his life less miserable. His overreliance on psychotropic drugs is probably exacerbating the issues. Bickle, meanwhile, takes a night job when he suffers from insomnia. He makes a lot of money doing it and, at least in the earlygoing, seems to maintain his composure despite acknowledging that he goes through tough areas and has had to deal with problematic customers. Mainly, I got the vibe that despite Bickle's increasingly dangerous and frightening behavior, likely stemming in part from PTSD and his philosophy regarding the NY he encounters late at night, that he was actively attempting and was even equipped to do something to rectify his problems. Fleck just seems like he's in such a bad way that it would be very difficult to ameliorate his circumstances.
It's cool that you bring up the empathy aspects as well. To me, you have two threads with Bickle. Unlike with Fleck, the capacity for violence, the building resentment seem oddly diffuse and impersonal. Maybe the whole point with targeting Pallantine was meant to be some sort of vengeance against Cybill Shepherd for rejecting him. But I really didn't see it that way. And I don't think the movie pushes that narrative. Once he lumps Cybill in with all the other "people in hell," he doesn't even seem to think about her in any substantial away or at least as far the film conveys. We get the insight that Bickle sees the streets of NY as a sinful sewer, but, interestingly, we don't even see anyone do anything really bad toward him personally. With Fleck, we see him have endure personal affronts and abuse at every turn.
Wizard (Peter f'n Boyle, RIP) and the co-workers are even pretty nice to Bickle, despite maybe thinking he's a bit strange. Hell, you even got the sense, with Cybill, that Travis was one fewer stupid decision away from becoming involved with a beautiful, intelligent woman.
So the other thread besides the intentions toward Pallantine (if they represent a more anarchic, horrific violence that Bickle is capable of toward innocents) is the contrast with Iris' situation. His being rightly horrified by the life she is trapped in and his desire to save her from it definitely added a different element. So if Fleck ultimately perpetrates violence against individuals he felt wronged him, Bickle ends up perpetrating violence against those who are exploiting and abusing a young girl.
With Joker, things almost play out like Arthur sees many people doing awful shit to him and then falls into evil with the notion that he will do far worse against anyone who has done him a wrong, regardless of the degree of the offense. With Taxi Driver, Bickle sees terrible shit (but not happening to him personally) and feels the drive to do something terrible as well. Frankly, I know the film ends with the notion that Travis, in spite of being perceived as a hero by the city, is still a ticking time bomb. But I found myself hoping the character would be able to rehabilitate himself.